1. Field of the Invention
The present invention relates to a magnetic recording medium having a high recording capacity, a high access speed and a high transmission speed. In particular, the present invention relates to a magnetic recording medium for data-backup, which records and reads data with a reading head comprising a magnetoresistance element (hereinafter referred to as “MR head”).
2. Prior Art
Magnetic tapes have various applications such as audio tapes, videotapes, computer tapes, etc. In particular, in the field of magnetic tapes for data-backup, a tape having a memory capacity of several ten GB or more per one reel is commercialized with the increase of the capacity of hard discs to be back-upped. Therefore, it is inevitable to increase the capacity of the tapes for data-backup. It is also necessary to increase the traveling speed of a tape and a relative speed between the tape and a head to increase an access speed and a transfer speed.
To increase the capacity of a tape for data-backup per one reel, it is necessary to increase a recording density in a width direction with decreasing a track width (a signal pattern width on the tape) to 15 μm or less in addition to the prolongation of a tape length per one reel with decreasing the thickness of the tape and the reduction of thickness demagnetization and thus the decrease of a recording wavelength with reducing the thickness of a magnetic layer to 0.3 μm or less.
When the thickness of the magnetic layer is reduced to 0.3 μm or less, the durability of the magnetic recording media tend to deteriorate. Therefore, at least one primer layer is provided between a non-magnetic support and the magnetic layer. When the recording wavelength is shortened, the influence of spacing loss between the magnetic layer and the magnetic head increases. Thus, if the magnetic layer has large projections, a half width value of output peak (hereinafter referred to as “PW50”) may increase or the output may decrease due to the spacing loss so that an error rate may increase.
Since a leaking magnetic flux from the magnetic recording medium decreases when the recording density in the width direction is increased with decreasing the track width to 15 μm or less, a MR head, which can achieve a high output even at a minute magnetic flux, is used as a reading head.
Examples of the magnetic recording media used with the MR head are disclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In these publications, the skew of output from the MR head is prevented with controlling the magnetic flux of the magnetic recording medium (a product of a residual magnetic flux density and a thickness), or the thermal asperity of the MR head is suppressed with reducing dents on the surface of the magnetic layer to a specific value or less.
The conventional magnetic head uses a chip itself, which is a laminate of a magnetic induction type head for recording and a magnetic induction type head for reading. On the other hand, the MR head 20 is combined with the magnetic induction type head 21 for recording and embedded in the slider 22, as schematically shown in FIGS. 2, 2a, 3 and 3a. In FIGS. 2a and 3a, numeral 20a stands for the MR element, 21a and 21b for magnetic elements composing the recording head 21, 21c for a reading gap, and 23 for a shielding member. The MR head is embedded with receding from the slider surface 22a by about 25 nm.
That is, the conventional head consists of a very small chip, and thus the magnetic tape runs over the head as if a knife edge bites the tape, while, in the case of the MR head, the magnetic tape 30 runs with contacting to the slider 22, since the MR head 20 is embedded in the large slider 22 with receding from the slider surface (as shown in FIGS. 2, 2a, 3 and 3a). The magnetic tape 30 and the MR head 20 are in contact with each other as if the magnetic tape 30 expands towards the MR head 20. Since the contacting state of the MR head with the magnetic tape is very different from that of the conventional magnetic head, properties required for the magnetic tape change completely in connection with the decrease of the spacing loss.
Furthermore, since the MR element 20a of the MR head 20 comprises a very thin film, it is easily worn out. As shown in FIGS. 2a and 3a, usually a pair of the MR head 20 and the recording head 21 are used so that the magnetic tape can be recorded and read when it runs either in a forward direction or in a backward direction. Furthermore, a plurality of pairs of the MR head and the recording head are provided as shown in FIG. 2 so that a plurality of tracks can be recorded and read at the same time.
In addition, since the MR head has a very narrow track width, a servo-signal is provided for the tracking servo of the MR head.
The track servo system includes a magnetic servo system and an optical servo system. The former system magnetically records a servo-band in the magnetic layer and reads the servo-band to carry out servotracking, while the latter system forms a servo-band comprising depression arrays with laser irradiation, etc. and optically reads the servo-band to carry out the servotracking. Besides the above systems, the magnetic servo system includes a system in which magnetism is imparted to the backcoat layer, and a magnetic serve-signal is recorded in the backcoat layer, and the optical servo system includes a system in which an optical servo signal is recorded in a backcoat layer with a material that absorbs light.
To keep up with the increase of the traveling speed of the magnetic tape and the relative speed between the tape and the head, it is necessary to travel the tape at a high speed while tracing the servo-signal. However, if a coefficient of friction of the magnetic layer or the backcoat layer against a slider material (for example, alumina/titania/carbide, etc.) or a material of guide rollers is insufficiently optimized, the magnetic tape meanders so that the tracking may deviate (off-track), PW50 may increase or the output may decrease. Therefore, the error rate may increase.